Much public discussion in Britain in recent weeks has focused on the government's plans to acquire a successor to the Trident nuclear-weapons system. This is clearly a vital issue, but it must also be seen in the context of an assessment of Britain's security needs in the 21st century as a whole. In this light, the plan to build two enormous aircraft-carriers is also a revealing sign of strategic thinking and priorities.

In June 2006, following its study of the future of Britain's nuclear weapons, the House of Commons defence select committee took the unusual step of inviting the ministry of defence to make a full and open debate possible. The ministry had earlier taken the extraordinary decision simply to refuse to give evidence to the committee, even though the government has claimed want a public discussion on the future of Trident and its possible replacement.

As the defence committee's summary explains:

"A genuine and meaningful debate is only possible with the active participation of the Ministry of Defence (MoD). The public should know what decisions will be required, when they must be taken and implemented, and what factors are driving consideration of the issue now. We call upon the MoD to engage fully in our forthcoming inquiries into the future of the UK's strategic nuclear deterrent. We hope the MoD will make a substantive response to this report and that it will address openly the issues we have raised."

The government appears to be giving contradictory signals: it claims to want an open debate, yet it is echoing the long-standing example of its predecessors in refusing to be explicit about its military posture - and about two key policy issues in particular.

Paul Rogers is professor of peace studies at Bradford University, northern England. He has been writing a weekly column on global security on openDemocracy since October 2001.

The first is the extent of Britain's reliance on the United States; not just in terms of missiles and other technologies, but of the extent of Britain's independence (if any) from United States policy (the latter is considered in openDemocracy by Dan Plesch, "Britain's choice: nuclear weapons or foreign policy", 11 July 2006).

The second issue is the question of what Britain's nuclear weapons are for. Three questions arise here:

are nuclear weapons an ultimate last-ditch deterrent, only to be used against some enemy threatening to destroy Britain in a nuclear attack?

does their utility extend far more widely – does, in fact, Britain still maintain a policy of first use, and therefore envisage using nuclear weapons in a regional conflict (a "small nuclear war in a far-off place")?

is the government prepared to be open about these possibilities?

An earlier column in this series included evidence that the answers to the second and third questions are "yes" and "no" respectively (see "Britain's nuclear-weapons fix", 29 June 2006). But these answers do not come from the government itself: all too often it cites considerations of national security that make no sense in themselves and contrast with (for example) the openness of American nuclear targeting policy, especially during the cold-war era.

If Britain does replace Trident, as many political signals indicate it will, this entails a commitment to remain a nuclear power for the next forty years or more. The implication is also a firm acceptance that a multi-nuclear world will also continue for at least several decades to come (at present, the number of nuclear-armed states is still small – the United States, Russia, China, France, Britain, Israel, India and Pakistan; North Korea may also have a handful of crude nuclear devices).

Those supporting retention of nuclear weapons argue that an uncertain global environment carries the risk that an enemy power will emerge to threaten the country, even if that might only happen in the distant future. If that is the rationale, developments in North Korea make it a much more immediately compelling argument for South Korea, Japan, Taiwan and Australia, and Britain could in no way criticise any of these countries if it took the nuclear option.

The long tail

Any public debate worth the name should consider such arguments before the Trident-replacement decision is formally made. A lesser-known aspect of Britain's military policy will also be decided on in the next year or so. It too will point Britain's defence posture in a particular direction: cementing a firm alliance with the United States as it embarks on its "long war" to try to maintain control of an uncertain and frequently anti-American world.

This proposal is to build the two largest and most powerful surface warships ever to see service in the Royal Navy. These 65,000-tonne Queen Elizabeth-class aircraft carriers are currently in the design stage. If they are constructed and enter service around 2016, they will give the Royal Navy a global power-projection capability that will exceed anything it has possessed for more than forty years; they will also be superior to anything deployed by any other country except the United States.

In the early cold-war years, when many people in Britain still thought there were three rather than two superpowers, the Royal Navy had large fleet carriers such as Eagle and Ark Royal that sailed the world, carried tactical nuclear weapons, and supported British governments as they sought to prolong Britain's world role. The post-Suez era of decolonisation and relative economic decline enforced a certain adaptation to new realities; the idea of a middle-ranking power maintaining such forces finally evaporated in the early 1970s as the carriers were scrapped, to be replaced by three much smaller Invincible-class ships that served mainly in the anti-submarine role.

In a decision that has attracted little controversy so far, the Labour government of Tony Blair announced during its first term that these would be replaced in due course with two carriers - each more than three times the size of Invincible and its sister ships and larger even than the old Eagle. The Queen Elizabeth-class carriers will not match the size and power of the American 100,000-tonne Nimitz-class super-carriers – but they will not be far behind, they will certainly enable the United Kingdom to "ride the pillion" of the United States in its military posture.

That posture is very much about fighting the long war, with an emphasis on maritime power-projection, special forces, and taking the war to the enemy. It has evolved rapidly since the 9/11 attacks: almost five years on, Britain has been deeply implicated in global changes that have seen two regimes terminated; more than 50,000 civilians killed in Afghanistan and Iraq; more than 100,000 people detained without trial; and the al-Qaida movement remaining as active as ever.

In addition to his weekly openDemocracy column, Paul Rogers writes an international security monthly briefing for the Oxford Research Group; for details, click here

The Pentagon, far from rethinking its policies in light of these results, has reinvented the "war on terror" as the "long war", all the while retaining its essential commitment to preserve the United States's position as the world's only superpower. But even superpowers like to have allies; and Britain in the 2020s – with its renewed nuclear forces and its two new "not-quite super-carriers" – will be a useful partner in future ventures abroad.

Britain itself, however, will be obliged to live with the combination of a Trident replacement and the two new carriers in the context of a tight defence budget and military forces that are a fraction of those of the United States. The effect will be to channel the UK security outlook into a particular paradigm: one of trying to control threats rather than understanding their causes and facing up to them at their source (see Paul Rogers, Losing Control: Global Security in the 21st Century, Pluto Press 2002).

In such circumstances, Britain's potential for supporting United Nations peacekeeping operations will be squeezed, as indeed will any prospect of abandoning support for the US's requirement of global dominance. Moreover, just as Britain's nuclear forces (current and future) are dependent on US technologies and equipment, so the carriers will be dependent on the new American F-35 joint strike fighter, the immensely expensive plane that will be deployed on the ships.

Many people are arguing for a proper debate on Britain's attitude to international security, one that should address some of the most severe and unavoidable 21st-century problems: the increasingly bitter global socio-economic divide, the potential for greater conflict over oil supplies, and what may be the biggest issue of all – climate change.

If such a debate does become possible, and if it is initially centred on Trident, than that will be a helpful start. But it will be essential to widen its scope, and ask whether Britain really should be building its largest warships ever. Are they really an appropriate response to the diverse security challenges of the 21st century?